1. Field
This disclosure relates to display systems, more particularly to display systems that require an external light source.
2. Background
Many products such as cameras, personal digital assistants (PDAs), electronic picture frames, laptops, portable electronic games, etc., employ display systems to convey information to a user. There are two general classes of these display systems; those that inherently provide a light source and those that require an external light source. Those that inherently provide a light source include cathode-ray tubes (CRTs), light-emitting diode (LEDs), incandescent displays and plasma displays. Those that require an external light source include liquid crystal displays (LCDs), digital micro-mirror devices (DMD(trademark)), and other micromechanical displays. Embodiments of the invention, as will be discussed later, also applies to discrete indicators such as LEDs and incandescent bulbs used in environments where power consumption is more important than cost.
Optical recording systems include such common appliances as video and still cameras. They record images or data corresponding with those images as received by the optics of the system. Many of these systems, especially those using digital technology, provide a separate viewfinder or display for the user.
Typically, the separate viewfinder is electronic, as opposed to the optical viewfinder through which a user looks to frame the subject. Optical viewfinders are generally found on every type of camera, be they still or video cameras, digital or analog. The electronic viewfinder recreates the image on a small display, allowing the user the chance to see the image in another manner before recording the scene. This viewfinder typically includes some sort of screen, usually an LCD (liquid crystal display) screen, and the appropriate electronics that accompany the screen.
There are two types of LCD display systems, reflective and backlit. Reflective LCDs place a mirrored surface behind the LCD and depend on the reflected ambient light to illuminate the image. With reflective designs, light must pass through the LCD, reflect off the mirrored surface, and pass back through the LCD a second time to the user. Due to the inherent optical transmission losses of LCDs, as much as 60% for a color LCD, reflective systems provide low contrast images that cannot be seen at all in dark environments. The alternate method, which is more expensive in both cost and power, is to use backlit screens. Backlit designs provide a light source that does not depend on ambient light. Backlit systems work well in dark environments but provide low contrast images when the ambient light is bright relative to the backlight source. This patent describes a technique that combines the low power and bright ambient viewing features of reflective designs with the high contrast and dark environment viewing features of backlit designs.
On some cameras screens allow the user to view and frame the current scene in real-time for image capture, to display option menus and status information, and to review images stored in memory, acting as a display. Physical controls on the camera (typically buttons and cursor pads) allow the user to determine what is displayed on the screen: real-time images, menus, status information, stored images, etc. To minimize the number of physical controls on the device, menus are displayed on the screen. The user may employ a simple set of cursor and selection controls to identify the desired menu options, such as reviewing the images, saving them or deleting them.
In either case, the camera must provide illumination to the screen to allow the user to view the images. Using a backlight for such a screen consumes a relatively large amount of battery power, requiring the batteries to be replaced or recharged more frequently than is convenient. In addition, the power of the backlight provided to the screen may not overcome the ambient illumination. For example, the user may try to review images in the camera, or record scenes, on a very bright day. The surrounding light may over power back light and the user will not see the images on the screen very clearly.
This problem could occur with any device with a display that is not self-illuminating. Self-illuminating displays would include such things as plasma and cathode-ray tube (CRT) displays that provide illumination as part of the process that actually forms the image. Displays that are not self-illuminating would include those with any kind of separate light source, such as LCD displays, either reflective or transmissive, digital micromirror displays (DMD(trademark)), and the like. Examples of such devices may include video cameras, still cameras, video game players, music players, and personal digital assistants (PDAs).
Therefore, it would seem desirable to have some method or apparatus to provide boosting power to the illumination of the screen, as well as reduce battery consumption.